The carboxylic acid chloride is usually produced through a reaction of a chlorinating agent such as phosgene, oxalyl chloride, phosphorus trichloride, phosphorous pentachloride, thionyl chloride or the like with a carboxylic acid. On this occasion, when an amide type compound is added as a catalyst in order to improve a reaction rate, the chlorinating agent is reacted with the amide type compound to form a Vilsmeier reagent type compound (so-called chloroiminium salt), which serves as active species of a synthesis reaction of an acid chloride. However, when these Vilsmeier reagent type compounds remain, they may often cause a side reaction or a coloring in a subsequent step due to their high activity.
Although depending on a kind of the starting carboxylic acid or a kind of the resulting carboxylic acid chloride, the Vilsmeier reagent type compound is commonly phase-separated to settle out in the form of a brown or black tar at a bottom of a reactor after the completion of the reaction. Therefore, the Vilsmeier reagent type compound is separated through a decantation or the like, and then the carboxylic acid chloride is isolated through distillation-purification. Thus, the carboxylic acid chloride can be easily separated from the Vilsmeier reagent type compound in general, so that there is no problem. Further, even when the Vilsmeier reagent type compound is insufficiently separated, there is known a method comprising polymerizing the Vilsmeier reagent type compound through heat treatment and purifying it by utilizing a boiling point difference or a solubility difference.
However, the resulting carboxylic acid chloride may lack a thermal stability or be hardly distilled in practice due to a high boiling point thereof. Further, the distillation may have to be avoided on an industrial scale, because it overloads a vacuum unit in view of ability and material thereof. In such a case, since the distillation-purification cannot be carried out, the reaction mixture including the dissolved Vilsmeier reagent type compound which cannot be completely separated, i.e., as it is a crude mass, must be used. In that case, there must be a problem such as the side reaction or the coloring due to the Vilsmeier reagent type compound.
Furthermore, some starting carboxylic acids have a high melting point depending on their type and can be often reacted only in the presence of a solvent, and the above-mentioned phase separation of the Vilsmeier reagent type compound is seldom observed. Therefore, the Vilsmeier reagent type compound is compulsorily phase-separated by properly adding a poor solvent such as hexane or the like, or a mixture including all of the Vilsmeier reagent type compound must be used without the distillation.
When the solvent such as hexane or the like is used, it is required to be used in about twice the amount of the carboxylic acid chloride and makes a volumetric efficiency deteriorated, and further the separation and recovery of the solvent make the cost higher. On the other hand, when there is no problem with equipment for the distillation, the distillation may be carried out while including the Vilsmeier reagent type compound, but there is a problem in that a skeleton balance is deteriorated to lower a yield or the like.
As a method for solving the above-mentioned problems, there is known a method using the chlorinating agent in an amount of slightly lower than a theoretical amount based on the carboxylic acid to leave the small amount of the starting carboxylic acid in order not to leave the Vilsmeier reagent type compound. However, it is difficult to precisely adjust the charging amount on the industrial scale, and therefore an equipment cost increases for carrying out. Especially, since phosgene industrially used as the chlorinating agent is supplied as a gas, it is more difficult to adjust the charging amount.
Moreover, when the starting material is a dicarboxylic acid, a reaction system is sophisticated. That is, a shortfall of the chlorinating agent is not equal to the remaining amount of the starting carboxylic acid, and an acid anhydride may be formed depending on a kind of the monoacid chloride or the dicarboxylic acid. Therefore, the method for not leaving the Vilsmeier reagent type compound by adjusting the amount of the chlorinating agent to be charged cannot be carried out.
In addition, there are known a method for improving a hue by contacting with chlorine after or during the reaction (JP-A-2002-363130) and a method for improving a hue by contacting with hydrogen chloride after or during the reaction (JP-A-2003-509393) and thereby the hue is improved, but the Vilsmeier reagent type compound is not removed and therefore the risk of the side reaction cannot be eliminated.